1. Field of the Invention
The present invention relates to a new and improved stylus probe that combines contact and capacitance capability for measuring surface characteristics such as the dimensions and contours of a workpiece requiring a probe having a high degree of sensitivity, resolution and an exceptional degree of adaptability as well as the ability to deflect elastically without any permanent or plastic deformation or bending of the probe.
2. Summary of the Prior Art
Two and three dimensional co-ordinate measuring machines which comprise a head supported for two or three-dimensional motion relative to a workpiece are known in the art. The movable head supports a stylus probe which is generally in the form of a straight rod with a small contact ball at the tip disposed away from the head. The machine includes a plurality of drive means for moving the head with the probe in two or three co-ordinate axes, and a monitoring means for instantly monitoring and recording the position of the probe with reference to the co-ordinate axes. A means is also provided for generating a signal when the probe comes into contact with the workpiece. When the probe is deflected, the driving motion of the probe is stopped and its position is recorded.
The co-ordinate measurements of a workpiece mounted to the co-ordinate measuring machine are determined by moving the head in one or more directions and reading the co-ordinate position of the probe relative to a given datum when the probe contacts the workpiece. State of the art co-ordinate measuring machines are capable of monitoring and recording probe positions to an accuracy of 0.1 mm.
Because the co-ordinate measuring machines of the prior art rely on physical contact to measure the co-ordinate positions of the workpiece surfaces, the operating sequence tends to be rather prolonged and time consuming. In addition, it is generally known that contact probes can be expensive to manufacture and are subject to wear and damage due to physical contact. Even a small amount of wear can introduce significant errors to the machine's accuracy capabilities.
In addition to the above, there are inherent difficulties and inaccuracies built into the prior art system and techniques for determining a contact position which tends to detract from the accuracy capabilities of the co-ordinate measuring machine as a whole. For example, setting the probe to an accurate zero starting position cannot be effected rapidly, as the probe must be brought into contact with the workpiece, and then carefully adjusted so that contact is maintained without probe pressure against the workpiece, or bending or deflection of the probe. A mere light touch contact of the probe against the workpiece is not normally sufficient to activate the stop signal. Rather, the stop signal is effected only after the probe has been deflected or in some way disturbed.
While it would be desirable to stop the motion of the head at the exact instant of probe contact, the contact signal does not normally occur immediately upon contact, and the drive mechanism cannot be stopped instantly upon receipt of the stop signal. Rather, the head continues to move by a minute amount relative to the probe tip before the stop signal can be effected. There is an even further minute initial movement before the stop signal effects a complete stop of the drive mechanism. The result is that the stylus probe is deflected under the operating force necessary to generate and transmit the stop signal, as well as to overcome the momentum of the drive means after the stop signal has been transmitted.
Since such probe deflection is inherent, it is normally taken into account in the measuring process. The amount of relative movement between the head and the probe tip is referred to as the "bending allowance," and is deducted from the distance measured by the machine. To measure from a surface where such a deflection has already occurred, a starting bending allowance must again be deducted from the distance measured or else the probe must be re-set to a proper zero starting point. Accordingly, consecutive measurements of different surface dimensions can be a relatively prolonged and time consuming procedure.
In order to prevent damage to the probe as a result of any bending action, many different types of stylus mounting systems have been devised to permit stylus deflection without permanent deformation. These include a calibrating means for determining the extent of deflection through a variety of complex probe mounting systems. These mounting systems normally comprise a 360 degree pivotal mounting base capable of measuring the extent of deflection and are not only intricate, complex, and costly, but do not achieve the degree of exacting measurements desired. Further, the mounting system can be damaged or knocked out of adjustment by the probe's impact with a workpiece surface, or merely by being accidently bumped by the operator.
As distinguished from contact probes, non-contacting capacitance probes have been utilized in prior art techniques for measuring conductive workpiece surface dimensions and characteristics. Capacitance probes afford the ability to determine the location of an adjacent workpiece surface without the need to contact the surface because a given capacitance value can be equated to a predetermined spacing, known as a "null point", between the probe and workpiece. Such probes have established a highly useful role in industrial manufacturing and in the machine tool industry for the characterization of surface properties and dimensions. These detectors are based on the employment of a variable capacitance due to the coupling of the detector with a conductive surface to be evaluated by techniques such as controlling the rate of an oscillator circuit, so that the frequency of the oscillator is directly determined and altered as a function of surface characteristics or distance from the probe.
Such capacitance probes are often utilized in combination with machine tools, such as a lathe or a milling machine for determining conformity of a machined workpiece with specifications before the workpiece is removed from the machine tool. Any incomplete machining detected by the capacitance probe can then be corrected before the workpiece is removed from the machine tool.
Most of the prior art practices utilizing capacitance detecting probes are based on the use of probes that incorporate a plurality of capacitor detectors so that the differences in capacitance, as measured by the various detectors, is a function of the workpiece surface characteristic sought.
U.K. Patent No. 2,100,441, to Wofendale, teaches a number of methods of determining the contour of an unknown conductive surface or a diameter of a bore utilizing a non-contacting probe, which comprises a probe having a plurality of capacitor sensors positioned at or near the probe tip. Differences in the measured capacitance between the workpiece surface and the various sensors provide data corresponding to the spacing between the workpiece and the sensors. Such capacitance probes having multiple sensors as taught in Wofendale usually comprise a cylindrical body portion such as a tubular body with the multiple sensors exposed through one or more side wall portions, and accordingly do not have any significant degree of compliance or flexibility. Wofendale does not teach or suggest a flexible metallic tubular stylus probe that can be used as both a capacitance and contact probe. He also does not teach a probe wherein vibrations, caused by a hard contact with a workpiece, can be dampened with the use of an elastomer in direct contact with a tubular member of the probe. Unlike the present invention, Wolfendale does not teach interchanging both capacitance and contact probe capabilities. Nor does he teach or suggest the use of an environmental probe to give more accuracy to the ultimate capacitance data.
U.S. Pat. No. 4,816,744, to Papurt, et al., teach a process and apparatus for measuring an inside dimension of a workpiece, such as a bore diameter. The apparatus positions a single workpiece surface, and then moves the probe to an equal null point adjacent to the other workpiece surface such as diametrically across a bore and then measures the distance of the probe movement with a laser interferometer. The desired distance or diameter is then determined to be the distance traveled by the probe plus the two stand-off distances as established by the null point capacitance. Since contact with the workpiece is not intended, the probe depicted in the patent is a short and rigid stylus probe which does not appear to have any conformity or flexibility characteristics. Papurt, et al., do not teach or suggest a stylus probe that can be used as both a capacitance and contact probe. Further, Papurt, et al., do not teach the use of an elastomer in direct contact with a tubular member of the probe, the elastomer having a dampening effect on the vibrations of the stylus probe. Nor does he use an auxiliary environmental probe to measure environmental interferences. Papurt, et al., do not teach or suggest a probe that can be used in both contact and capacitance modes.
U.S. Pat. No. 4,333,238 to McMurtry, teaches a coordinate measuring machine adapted to move a probe relative to a workpiece to be measured, the measurement being determined by an electronic sensor on the carriage, actuated by deflection of the probe when it contacts the workpiece. The invention provides an accelerometer to determine the acceleration and, derivatively, the deflection of the probe, the probe's defection measured in micrometers, with a computer arranged to determiner the true measurement by adjusting for the deflection and making a proper calculation. McMurtry does not suggest or teach a stylus probe deflection capability of up to 45.degree. nor does McMurtry teach the use of an elastomer to dampen the vibrations caused by the deflection of said probe. Further, McMurtry does not teach the use of an auxiliary environmental probe to more accurately measure capacitance nor does the McMurtry probe have both capacitance and contact capabilities.
U.S. Pat. No. 5,065,103 to Slinkman, et al., is an apparatus and method for generating microscopic scan data of C-V and/or dC/dV over a scan range. A scanning force microscope is provided with a voltage biased tip of tungsten which is scanned across an area to derive data. By analyzing voltage and capacitance data, a profile of material properties are determined, such as a plot of semiconductor dopant level across the scan area, carrier generation and recombination rates, and subsurface defects. Slinkman, et al., do not teach or suggest a stylus probe or method that can be used in either a capacitance or contact mode. He does not teach the dampening of probe vibrations by the use of an elastomer in direct contact a tubular member of the probe. Further, Slinkman, et al., do teach the use of an auxiliary environmental sensor in determining the accuracy of capacitance, but not the use of a highly flexible probe capable of a 45.degree. deflection without bending. Slinkman's teachings are not applicable to coordinate measurement machines.
U.S. Pat. No. 4,296,371 to Keizer, et al., provides a system for measuring shoe length of a stylus adapted to track a path along a surface of a video disc by positioning the tip of the stylus adjacent the surface if a substrate having a signal for effecting capacitive variations between the stylus and the substrate in a first position. The stylus is then tilted an angle of 0 degrees relative to the substrate, whereby the stylus and substrate are oriented in a second position relative to each other. The capacitive variations are then measured with a comparison made between the two stylus positions. Keizer, et al., do not teach or suggest a stylus probe that has the capability of both capacitance and contact functions, nor do Keizer, et al., teach the use of an elastomer in direct contact with the probe to dampen vibrations of the probe. Further, Keizer, et al., do not teach the use of an environmental probe to give greater accuracy in the measurement of capacitance. Keizer, et al., has no applicability to a coordinate measurement machine.
Since capacitance probes are not intended to contact the workpiece surface in order to make a measurement determination, there is no need to determine any bending allowance, and therefore, capacitance probes are normally rather inflexible structures rigidly secured to the motion machine such as a co-ordinate measuring machine or a machine tool. However, in normal use a probe may be subjected to occasional impact with the workpiece by accident, such as when the workpiece is being mounted to the measuring machine or machine tool, or accidently bumped by the operator when the probe is mounted to a machine tool. Therefore, while the probe is not intended to contact the workpiece and there is no need to determine any degree of deflection, it has become apparent that the stylus probe should have some degree of elastic flexibility so that it will not be permanently bent or knocked out of adjustment if it is accidently impacted or bumped.
In addition to the above considerations, the capacitance stylus probes of the prior art, particularly those having a single sensor element, have not had a significantly high degree of sensitivity and resolution, due to various factors. These factors are: 1) the capacitance effects on the stylus stem or portions of the probe other than the sensor itself, 2) stray capacitance and 3) environmental conditions such as temperature, humidity, atmosphere, and external electrical noise such as electromagnetic interference and radio-frequency interference, and the proximity of conductive bodies may affect the capacitance measured by the sensor itself, which will introduce errors into the measured capacitance being sought. Prior art capacitance sensors have not been compatible with the 0.1 mm accuracy capabilities of state of the art co-ordinate measuring machines.